Hazelnut growers in the Willamette Valley have been telling us what research now confirms: The herbicide 2,4-D drifting into the tree canopy delays nut drop (abscission) in hazelnut orchards. In this publication, you will learn the sources of 2,4-D drift, the environmental conditions that support it and how to reduce risk of 2,4-D drift in your orchard. Grower observation inspired this field research.
Herbicide 2,4-D is a synthetic auxin — a class of growth regulators. It is commonly applied to the base of hazelnut trees to manage suckers. As an herbicide, 2,4-D kills plants by causing xylem cells to grow out of control, disrupting their function.
Sucker management is an essential part of hazelnut production. It facilitates mechanical harvest, removes a potential source of disease organisms and increases yield.
2,4-D is a commonly used herbicide in many production systems in the Willamette Valley, the heart of the U.S. hazelnut industry. Hazelnut orchards are at risk of exposure to 2,4-D drift.
Studies by other researchers in grape, pear and apple indicate delayed fruit ripening after exposure to auxin herbicides.
Sources of drift
Hazelnut orchards may be exposed to damaging 2,4-D drift from applications within the orchard or from 2,4-D drifting in from surrounding crops. Drift can be primary — off-target spray particles at the time of application. Or, it can be secondary — herbicide movement after completion of an application. Volatilization is a common example of secondary drift.
We studied the effect of simulated 2,4-D drift on hazelnut abscission and yield in a 10-year-old hazelnut orchard. Weed and sucker management in the study orchard were consistent with grower standard practice.
Four basal-directed treatments spaced 28 days apart were applied to suckers between May and August during each year of the three-year study. The number and timing of applications were based on the use pattern permitted on the 2,4-D label and adopted by growers. Application rates represented 1-, 2- and 4-fold the maximum seasonal use of 2,4-D.
In a subset of treatments, 2,4-D drift at 1/10th and 1/100th of the field rate was simulated once yearly in the second half of July when nut filling occurs rapidly. Simulated drift was included in Years 1 and 2 of the study, but not in Year 3, to evaluate plant recovery from drift in the following season.
What we learned:
Abscission delay
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In mild cases, 2,4-D drift in hazelnuts caused symptoms such as leaf cupping, crinkling, and chlorosis of leaf margins (Figure 1).
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Severe cases of 2,4-D drift resulted in tissue necrosis and dead limbs. Dead limbs were only observed with 2,4-D drift at 1/10th of the field rate (0.6 mg L⁻¹) (Figures 2A and 2B).
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Simulated 2,4-D drift delayed hazelnut abscission, but only during the years when drift occurred (Figures 3A and 3B). No lingering effects on hazelnut abscission were observed in subsequent years.
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In Year 1, the time to reach 50% nut fall increased by approximately five days and 15 days for the two drift dosages, respectively. A similar response occurred in Year 2, with 2,4-D drift delaying nut fall by approximately five and eight additional days.
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When 2,4-D was applied directly to suckers (no drift), hazelnut abscission was unaffected, regardless of the rate used (1 to 4 times the annual rate).
Yield
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Simulated drift treatments had no effect on yield in Year 1 of the study.
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In Year 2, 2,4-D drift reduced yield by 18% and 37% for drift rates of 1/100th (0.06 mg L⁻¹) and 1/10th (0.6 mg L⁻¹) of the field rate, respectively, compared to the nontreated control.
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No impact on maximum yield was observed in Year 3, which did not receive simulated drift treatments.
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Drift reduction practices are essential for ensuring timely hazelnut abscission and maintaining high crop yields.
Mitigating 2,4-D drift in your orchard
Management practices
- Increase droplet size: Use drift reduction nozzles, such as Air Induction Under Leaf (AIUB) spray nozzles, to produce larger droplets that are less likely to drift off-target.
- Reduce spray pressure: Lowering spray pressure minimizes the formation of fine droplets, which can remain airborne near the crop and drift into the canopy.
- Monitor wind speed: Avoid spraying during calm conditions, which are often associated with temperature inversions that promote drift into the canopy. Similarly, do not spray when winds exceed 6 mph, as high wind speeds increase the risk of drift.
Environmental conditions
- 2,4-D volatilization is influenced by air temperature and humidity. It can volatilize at temperatures above 60°F, with the likelihood increasing significantly above 85°F, especially when humidity is low.
- Spray drift moves most efficiently at wind speeds greater than 6 mph.
Reducing the risk of 2,4-D drift in orchards
- Spray herbicides or agricultural products when wind speeds measure between 2 and 8 mph.
- Equip sprayers with drift-reduction nozzles, such as turbo air-induction nozzles, to minimize drift.
- Apply 2,4-D at lower pressures to create larger droplets and mitigate the risk of drift.
- Time applications earlier in the season, using 2,4-D before May 30, when air temperatures are lower and less conducive to volatilization.
- Apply 2,4-D during daylight hours to increase weed uptake and reduce the risk of thermal inversion, which is more common in the early morning or evening.
- Choose low-volatility 2,4-D formulations, such as acid or choline formulations, to reduce drift risk.
- Mild 2,4-D drift symptoms include chlorotic leaf margins, leaf cupping, and crinkling. Severe drift can cause tissue necrosis and dead limbs. A single drift at 1/100th of the field rate delayed hazelnut abscission, and exposure closer to harvest could cause greater delays.
- Consider alternative herbicides if drift is a concern. Refer to the PNW Weed Management Handbook for a list of registered options for sucker control in hazelnuts.
- Always follow pesticide label instructions. It is a violation of federal law to apply any herbicide inconsistently with its labeling.
Resource
Moretti, M.L. and L.L. de Souza. 2023. Hazelnut abscission is delayed by simulated drift of 2,4-D. Weed Science. 71: 506–513.
Use pesticides safely!
- Wear protective clothing and safety devices as recommended on the label. Bathe or shower after each use.
- Read the pesticide label—even if you’ve used the pesticide before. Follow closely the instructions on the label (and any other directions you have).
- Be cautious when you apply pesticides. Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use.
